1. Field of the Invention
The present invention relates to the modification of surfaces, in particular to the modification of fluorinated polymer surfaces so as to enable the deposition of non-fluorinated substances thereon. The invention may be especially useful in the field of thin film devices, such as thin film transistors.
2. Related Technology
Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semiconductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semiconductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semiconductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semiconductive material to accept, conduct and donate holes or electrons can be enhanced by doping the material.
For example, a p-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the highest occupied molecular orbital (HOMO) level of the semiconductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the lowest unoccupied molecular orbital (LUMO) level of the semiconductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form a thin film transistor (TFT). When an organic material is used as the semiconductive material in such a device, it is known as an organic thin film transistor (OTFT).
OTFTs may be manufactured by low cost, low temperature methods such as solution processing. Moreover, OTFTs are compatible with flexible plastic substrates, offering the prospect of large-scale manufacture of OTFTs on flexible substrates in a roll-to-roll process.
With reference to FIG. 1, the general architecture of a bottom-gate organic thin film transistor (OTFT) comprises a gate electrode 12 deposited on a substrate 10. An insulating layer 11 of dielectric material is deposited over the gate electrode 12 and source and drain electrodes 13, 14 are deposited over the insulating layer 11 of dielectric material. The source and drain electrodes 13, 14 are spaced apart to define a channel region therebetween located over the gate electrode 12. An organic semiconductor (OSC) material 15 is deposited in the channel region for connecting the source and drain electrodes 13, 14. The OSC material 15 may extend at least partially over the source and drain electrodes 13, 14.
Alternatively, it is known to provide a gate electrode at the top of an organic thin film transistor to form a so-called top-gate organic thin film transistor. In such an architecture, source and drain electrodes are deposited on a substrate and spaced apart to define a channel region therebetween. A layer of an organic semiconductor material is deposited in the channel region to connect the source and drain electrodes and may extend at least partially over the source and drain electrodes. An insulating layer of dielectric material is deposited over the organic semiconductor material and may also extend at least partially over the source and drain electrodes. A gate electrode is deposited over the insulating layer and located over the channel region.
An organic thin film transistor can be fabricated on a rigid or flexible substrate. Rigid substrates may be selected from glass or silicon and flexible substrates may comprise thin glass or plastics such as poly(ethylene-terephthalate) (PET), poly(ethylene-naphthalate) (PEN), polycarbonate and polyimide.
The organic semiconductive material may be made solution processable through the use of a suitable solvent. Exemplary solvents include mono- or poly-alkylbenzenes such as toluene and xylene; tetralin; and chloroform. Preferred solution deposition techniques include spin coating and ink jet printing. Other solution deposition techniques include dip-coating, roll printing and screen printing.
The length of the channel defined between the source and drain electrodes may be up to 500 microns, but preferably the length is less than 200 microns, more preferably less than 100 microns, most preferably less than 20 microns.
The gate electrode can be selected from a wide range of conducting materials for example a metal (e.g. gold) or metal compound (e.g. indium tin oxide). Alternatively, conductive polymers may be deposited as the gate electrode. Such conductive polymers may be deposited from solution using, for example, spin coating or ink jet printing techniques and other solution deposition techniques discussed above.
The dielectric material may be deposited by thermal evaporation, vacuum processing or lamination techniques as are known in the art. Alternatively, the dielectric material may be deposited from solution using, for example, spin coating or ink jet printing techniques and other solution deposition techniques discussed above.
If the dielectric material is deposited from solution onto the organic semiconductor, it should not result in dissolution of the organic semiconductor. Likewise, the dielectric material should not be dissolved if the organic semiconductor is deposited onto it from solution. Techniques to avoid such dissolution include: use of so-called “orthogonal” solvents for example use of a solvent for deposition of the uppermost layer that does not dissolve the underlying layer; or/and cross linking of the underlying layer.
The thickness of the insulating layer is preferably less than 2 micrometers, more preferably less than 500 nm.
It is known that the dielectric material may comprise a fluorinated polymer. Suitable fluorinated polymers include perfluoro cyclo oxyaliphatic polymer (CYTOP), perfluoroalkoxy polymer resin (PFA), fluorinated ethylene-propylene (FEP), polyethylenetetrafluoroethylene (ETFE), polyvinylfluoride (PVF), polytetrafluoroethylene (PTFE), polyethylenechlorotrifluoroethylene (ECTFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), perfluoro elastomers (FFKM) such as Kalrez® or Tecnoflon®, fluoro elastomers such as Viton®, Perfluoropolyether (PFPE) and a polymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV).
Fluorinated polymers are an attractive choice for the dielectric material, particularly in the field of organic thin film transistors (OTFTs), because they may possess a number of favorable properties including:—
(i) Excellent spin coating properties, for instance: (a) wetting on a wide variety of surfaces; and (b) film formation, with the option of doing multi-layer coatings.
(ii) Chemical inertness.
(iii) Quasi-total solvent orthogonality: consequently, the risk of the OSC being dissolved by the solvent used for spin-coating the dielectric is minimal, as is the risk of interaction with the solvent used to inkjet the gate electrodes.
(iv) High hydrophobicity: this can be advantageous because it results in low water uptake and low mobility of ionic contaminants in the fluorinated polymer dielectric (low hysteresis).
However, the use of fluorinated polymers as the dielectric material in OTFTs presents a number of challenges.
First, it is difficult to achieve good wetting of substances such as conducting polymers which are to be deposited on to the fluorinated polymer. For instance, in a top-gate organic thin film transistor (OTFT), conducting inks comprising one or more conducting polymers may be used to inkjet print the gate electrodes onto the insulating layer (more commonly used are metal colloid inks for printing the gate electrodes in top gate devices); however, this can be difficult when the insulating layer comprises a fluorinated polymer.
Second, adhesion of the substance, e.g. a conducting ink, to the fluorinated polymer surface may be unsatisfactory during and/or after post-deposition drying and annealing.
Third, a glue may be used when it is desired to encapsulate a device; however, achieving good adhesion between the glues that are most commonly used and a fluorinated polymer surface may be difficult.
A number of approaches have been taken in order to try to alleviate or solve one or more of these problems.
A first category of approach may be generally termed reactive treatments. In one example of such a treatment, a fluorinated polymer is treated using sodium in liquid ammonia or sodium naphthalene in THF, which act as strong reducing agents, resulting in the cleavage of C—F bonds and the formation of sodium fluoride, carbon radicals and carbanions. Defluorination can occur rapidly.
In a study, Ha and co-workers [Journal of Adhesion 33, No. 3, p. 169-84 (1991)] showed that sodium naphthalenide treatment of perfluoroalkoxy copolymer (PFA) introduced unsaturation to a depth of 112 nm, while oxygen-containing functional groups such as —OH, —C═O and —COOH were concentrated in the first few nm.
In another reported study, Marchesi at al [Journal of Adhesion 36, No. 1, p. 55-69 (1991)] treated various fluorinated polymers with sodium naphthalenide in THF at ambient temperature. They determined that the treated depth was within the range of 112-150 nm for PFA, FEP and PTFE.
These treatments have been reported to result in a “groove-like” topography [Lin et al., Journal of Adhesion Science and Technology 14, No 1, p. 1-14 (2000)].
In another study, Castello and McCarthy [Macromolecules 20, No. 11, p. 2819-28 (1987)] found that the reduction of PTFE by the potassium salt of the benzoin dianion in dimethyl sulfoxide (DMSO) results in a slight coloration of the PTFE surface and a considerable enhancement of the surface energy.
It has also been reported that although this treatment could successfully be applied to a range of fluoropolymers, Teflon® AF (a copolymer of TFE and perfluoro 2,2-dimethyl-1,3dioxole) was totally inert [Hung et al., Journal of Applied Polymer Science 55, No. 4, p. 549-59 (1995)].
It will be readily appreciated that wet-chemical reductive treatments typically involve the use of highly reactive chemicals, which may be dangerous or difficult to handle. The reductive treatments described above all involve use of a strong electron donor dissolved in a solvent: for instance, sodium in liquid ammonia, sodium napthalemide in tetrahydrofuran (THF), or the potassium salt of the benzoin dianion in DMSO. Reduction of the fluorinated polymer surface results in the release of fluoride anions, which are washed off as sodium fluoride or potassium fluoride, dissolved in the reduction liquid.
Further, such treatments may result in undesirable roughening of the fluorinated polymer surface. Yet further, such treatments may introduce contaminants that impact on the performance of devices such as OTFTs.
So-called plasma treatments represent a second approach to the modification of fluorinated polymer surfaces. It is known, for instance, that PTFE surfaces become water-wettable after short (10-20 second) treatments in either oxygen or argon plasma. It is also known that the adhesion of a vacuum deposited gold film on Teflon® increases after these treatments. Oxygen plasma is said to introduce C═O, —CH2, and —CHF groups onto the PTFE surface [Kinbara et al., Journal of Adhesion Science and Technology 7, No. 5 p. 457-66 (1993)].
In another study, Inagaki et al [Journal of Adhesion Science and Technology 3, No. 8, p. 637-49 (1989)] studied the effect of NH3-plasma treatments on PTFE surfaces. Water contact angles as low as 16 degrees were observed and it was reported that extensive defluorination had occurred, accompanied by the formation of carbonyl and amide surface groups.
The use of reactive silanes (e.g. 3-aminopropylethoxysilane) in a plasma process has also been investigated. It has been reported that the reactive silanes can be covalently coupled to the surface of inert fluoropolymers such as PTFE and may subsequently act as an adhesion promoter (“Cold Gas Plasma and Silanes”, by S. L. Kaplan, Fourth International Symposium on Silanes and Other Coupling Agents, Jun. 11-13, 2003, Orlando, Fla.).
The use of fluorosurfactants has been investigated in another general type of approach to the modification of fluorinated polymer surfaces.
For instance, it is known that Zonyl FSN, an adhesion promoter made by Du Pont, can help other films adhere to the fluoropolymer TFE AF. U.S. Pat. No. 5,403,437 discloses that by adding 5% ZFSNF to a photoresist solution and applying the photoresist on TFE AF at 70° C., coverage of approximately 98% can be obtained. Full coverage was said to be obtained by applying the photoresist solution twice.
Also, polymeric fluorocarbon surfactants such as poly(N-vinyldextranaldonamide-co-N-vinylperfluoroundecanamide), in which hydrophilic dextran oligosaccharides and hydrophobic perfluoroundecanoyl groups are incorporated sequentially onto a poly(vinylamine) backbone, have been shown to display stable surfactant adsorption and adhesion on PTFE surfaces [S. Wang et al., Macromolecules 37(9), p. 3353-3359 (2004)].
A problem associated with the fluorosurfactant methods is that the use of (liquid) fluorosurfactant molecules can potentially result in diffusion of the surfactant molecules into the active layers in electronic devices (e.g. the OSC layer in OTFTs).
Thus, there is a need for a surface modification technique for fluorinated polymer surfaces, which does not suffer from, or at least reduces the effect of the disadvantages associated with previous techniques.